The inquiry under the Conflict of Interest Code for MPs comes after Dawson conducted an initial review following a request by Conservative MP and leadership hopeful Andrew Scheer.

The commissioner also started an examination in January under the Conflict of Interest Act regarding the same trip following a request from Tory MP Blaine Calkins.

"We cannot tell you anything else at this time, as our office is required to conduct all investigations in confidence," Dawson's office said in a statement Monday.

Trudeau has come under fire from opposition parties for using the Aga Khan's personal helicopter to get to the island and for accepting such a gift from a longtime family friend.

Ethics Commissioner Mary Dawson prepares to appear at Commons committee, on Parliament Hill in Ottawa in June 2014. (Photo: Fred Chartrand/The Canadian Press)

The Prime Minister's Office did not offer an updated response to Dawson's second investigation on Monday, noting in a statement that Trudeau is happy to answer any questions the commissioner may have.

Both the Conflict of Interest Act and Trudeau's own ethics guidelines bar the use of sponsored travel in private aircraft, allowing only for exceptional circumstances related to the job of prime minister and only with the prior approval of the ethics commissioner.

Interim Conservative leader Rona Ambrose took a Caribbean vacation on a billionaire's yacht around the time members of her caucus were criticizing Trudeau for vacationing with the Aga Khan, but she insists she did not breach ethical guidelines.

"I followed all the rules," she said outside the Commons Monday. "I don't have anything to add to that."